Application of single exposure alternating aperture phase shift mask to form sub 0.18 micron polysilicon gates

ABSTRACT

In accordance with the objects of this invention, a new method of fabricating a polysilicon gate transistor is achieved. An alternating aperture phase shift mask (AAPSM) is used to pattern polysilicon gates in a single exposure without a trim mask. A semiconductor substrate is provided. A gate dielectric layer is deposited. A polysilicon layer is deposited. The polysilicon layer, the gate dielectric layer and the semiconductor substrate are patterned to form trenches for planned shallow trench isolations (STI). A trench oxide layer is deposited filling the trenches. The trench oxide layer is polished down to the top surface of the polysilicon layer to complete the STI. A photoresist layer is deposited and patterned to form a feature mask for planned polysilicon gates. The patterning is by a single exposure using an AAPSM mask. Unwanted features in the photoresist pattern that are caused by phase conflicts overlie the STI. The polysilicon layer is etched to form the polysilicon gates.

This is a Divisional application of U.S. patent application Ser. No.10/135,071, filed on Apr. 20, 2002, which is herein incorporated byreference in its entirety, and assigned to a common assignee.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

The invention relates to a method of fabricating semiconductorstructures, and more particularly, to a method of using an alternatingaperture phase shift mask to fabricate sub 0.18 micron polysilicon gatetransistors.

(2) Description of the Prior Art

Device shrinkage requires photolithographic enhancements to extend thecapabilities of the processes. One such enhancement is the use of phaseshift masks. Phase shifting masks are used when the desired feature sizeof an integrated circuit layer is on the same order of magnitude as thewavelength of light used in the photolithographic process.

In a typical photolithographic mask, layer features, or traces, areformed on the mask in an opaque material such as chrome. This chromelayer is formed overlying a transparent quartz substrate. Light is shownthrough this mask to expose a photosensitive material, commonlyphotoresist, as defined by the mask pattern. After the photoresist isdeveloped, the photoresist will reflect a copy or a reverse copy of themask pattern.

As the pattern features approach the wavelength of the exposure lightand the limits of the photolithographic equipment alignment andrepeatability, it is increasingly difficult to successfully transfer thepattern features. In practice, it is found that the current 248nanometer lithographic tools cannot reliably create patterns below 0.15microns.

One approach to extending the capability of the current lithographictechnology is the application of phase shifting masks. In a phaseshifting mask, an additional component is added to the chrome and quartzsystem. Either through the application of an additional transparentlayer or the through the removal of a portion of the quartz layer to aspecific depth, the optical properties are changed in a part of thetransparent (not covered by chrome) sections of the mask. Specifically,when light of the lithographic wavelength is shown through the mask, aphase shift is created between light waves that pass through the phaseshifted area and the light waves that pass through the non-phase shiftedarea. By shifting the phase of the light by 180 degrees, nodes, orcancellations of energy will occur at opaque boundaries between thephase shifted and non-phase shifted areas. This principle is used tocreate more sharply defined boundary conditions during thephotolithographic exposure. Sharper definition leads to improved patterntransfer.

The phase shifted mask principle has been applied using an alternatingaperture phase shift mask (AAPSM) approach. In AAPSM, the transparentmask sections are alternated between 0 degrees (non-shifted) and 180degrees (shifted). The resolution limit of periodic line patterns can beimproved by a factor of two using AAPSM. However, application of AAPSMto random lines is limited and made difficult due to phase conflicts atdirect boundaries between phases where no opaque line separates thephase areas. Techniques that have been employed to eliminate theseproblems include utilizing a multiphase boundary (such as 60 degreessteps) and using a binary trimming mask to expose the unwanted areas.The use of multiphase AAPSM causes problems in mask fabrication andcause space constraints.

The use of the prior art binary trim mask method is illustratedbeginning in FIG. 1. A cross section of a prior art device is shown. Asemiconductor substrate 10 is provided. Shallow trench isolations (STI)14 are formed in the semiconductor substrate 10. The STI 14 separate andisolate active areas. A gate oxide layer 18 is formed overlying thesemiconductor substrate 10 and the STI regions 14. A polysilicon layer22 overlies the gate oxide 18. Note that the top surfaces of the STIregions 14 are co-planar with the top surface of the semiconductorsubstrate 10.

Referring now to FIG. 2, a top view of a planned memory cell 38 isshown. In this cell, two p-well active areas 30 and two n-well activeareas 32 are shown. Polysilicon traces 34 overlie the active areas 30,32 to form transistors 36. Six transistors are formed in the memorycell. Note that the polysilicon layer 34 also provides commonconnectivity for the cell.

Referring now to FIG. 3, a first mask 39 used for the first step ofpatterning the planned polysilicon traces is shown. Chrome traces 42 areformed with the minimum lithographic width L1 for the typical process.This mask represents the binary image of the layout of the polysiliconlayer for the cell. Non chrome areas are transparent. For example, theminimum width L1 is 0.18 microns. This same minimum width L1 is usedeven in the transistor areas 36. Following the application of aphotoresist layer overlying the polysilicon layer 22 of FIG. 1, a firstexposure is performed using the chrome mask 39 of FIG. 3. Following thefirst exposure, the photoresist is not developed.

Referring now to FIG. 4, a second mask 41 is shown. This mask utilizesan alternating aperture phase shift mask (AAPSM) approach. The secondmask 41 is a dark field mask so that most of the area is chrome 51 withopenings 46, 50. Non-phase shifting areas 46 are formed parallel tophase shifting areas 50. Both the non-phase shifting areas 46 and thephase shifting areas 50 allow light passage. Both non-phase shiftingareas 46 and phase shifting areas 50 overlap the chrome pattern of thefirst mask 39 of FIG. 3. The space L2 between the non-phase shiftingareas 46 and the phase shifting areas 50 is smaller than the minimumwidth L1 of the chrome mask 39. In the art, the minimum space L2 betweenphase areas is between about 0.09 microns and 0.15 microns. This secondmask 41 is used in a second exposure of the photoresist and acts a trimmask because the second exposure trims back some of the unexposedphotoresist. The presence of the non-phase shifting areas 46 and thephase shifting areas 50 allows a narrower line width to be formed in thephotoresist.

Referring now to FIG. 5, the top view of the resulting memory cell 47 isshown. After the second exposure step, the photoresist layer isdeveloped. The polysilicon layer is etched to form traces 54. Note thatthe polysilicon layer 54 is narrower in the gate regions where itoverlies the active areas for the p-well 30 and n-well 32. The two maskAAPSM sequence allows shorter gate length transistors to be formed.

Referring to FIG. 6, a cross sectional view of the finished prior artdevice is shown.

There are three drawbacks to the process of the prior art. First, thisprocess requires two masking steps that must be very carefully aligned.This is both expensive and time consuming. Second, because thepolysilicon layer 54 is used as both the interconnecting layer and thegate layer, the memory cell must be made relatively large. Thepolysilicon layer 54 connectivity points must be sufficiently spacedfrom the active areas 30, 32 so that parasitic transistors are notcreated. Third, if the first and second masking steps are not perfectlyaligned, open or high resistivity connections can be created.

Referring now to FIG. 7, a representative top view of a transistorcreated in the prior process is shown. The polysilicon layer 54 overliesthe p-well active area 30 to form a transistor. A misalignment betweenthe first chrome mask 39 and the second phase shifting mask 41 hascaused the connectivity section of the polysilicon to be offset 58 fromthe gate. Though this is not an open circuit, the offset will cause alarger than normal gate resistance that will diminish deviceperformance.

Several prior art approaches concern methods to create or to use phaseshifting masks in the manufacture of integrated circuits. U.S. Pat. No.5,468,578 to Rolfson teaches a process to form alternate aperture phaseshift masks (AAPSM) where two E-beam writes are used. A four step methodis disclosed to: (1) identify phase conflict areas in the mask data; (2)connect together adjacent planned transparent areas in phase conflictlocations; (3) deposit and pattern opaque material to form transparentareas; and (4) phase shifting areas by either etching down transparentareas or by adding phase shifting material in every other transparentopening and in all connecting areas. U.S. Pat. No. 5,670,281 to Daidiscloses a method to form phase shift masks that eliminates bridgingproblems due to phase conflicts at the ends of parallel features. Finetips, that is, triangle shaped ends, are used to eliminate phaseconflict problems. Methods for forming 180 degree phase shift materialusing either subtraction of transparent material or addition andpatterning of a phase shift layer are disclosed. U.S. Pat. No. 5,935,740to Pierrat discloses a process to form alternate aperture phase shiftmasks where multiple layers of light transmitting, phase shiftingmaterial are used. Each phase shifting layer is optimized to shift theincident light 60 degrees. By patterning the phase shifting layers,complementary phase shifting patterns are created in adjacent circuitareas. U.S. Pat. No. 5,582,939 to Pierrat teaches a process to formphase shift masks where bump defects in the phase shifting material areremoved.

SUMMARY OF THE INVENTION

A principal object of the present invention is to provide an effectiveand very manufacturable method of fabricating a polysilicon gatetransistor in the manufacture of integrated circuits.

A further object of the present invention is to provide a method of afabricating polysilicon gate transistor where an alternating aperturephase shift mask (AAPSM) is used to pattern the polysilicon gates.

A yet further object of the present invention is to provide a method offabricating a polysilicon gate transistor where an alternating aperturephase shift mask (AAPSM) is used to pattern the polysilicon gates usinga single alignment and exposure step.

Another yet further object of the present invention is to provide amethod of fabricating a polysilicon gate transistor where an alternatingaperture phase shift mask (AAPSM) is used to pattern the polysilicongates where unwanted pattern features in the lithographic process causedby phase conflicts fall over shallow trench isolation (STI) regions andare therefore not transferred to the polysilicon layer.

Another further object of the present invention is to provide a methodof fabricating polysilicon gate transistor wherein tungsten silicide ora combination of titanium and titanium nitride is used to forminterconnect lines to the polysilicon gate transistor.

Another object of the present invention is to provide a method offabricating an alternating aperture phase shift mask (AAPSM) for use informing polysilicon gate transistors in the manufacture of an integratedcircuit device.

Another further object of the present invention is to provide a methodof fabricating an AAPSM where the phase shift layer is adjacent to theopaque layer used to define the polysilicon gate and overlaps the activearea onto the STI region so that unwanted patterns due to phaseconflicts fall onto the STI regions.

In accordance with the objects of this invention, a new method offabricating a polysilicon gate transistor is achieved. A semiconductorsubstrate is provided. A gate dielectric layer is formed overlying thesemiconductor substrate. A polysilicon layer is deposited overlying thegate dielectric layer. The polysilicon layer, gate dielectric layer, andsemiconductor substrate are patterned to form trenches for plannedshallow trench isolations. A trench oxide layer is deposited overlyingthe polysilicon layer and thereby filling the trenches. The trench oxidelayer is polished down to the top surface of the polysilicon layer tocomplete the shallow trench isolations. A photoresist layer is depositedoverlying the polysilicon layer and the shallow trench isolations. Thephotoresist layer is patterned to form a feature mask for plannedpolysilicon gates. The patterning is by a single exposure using analternating aperture phase shift mask (AAPSM). Unwanted features in thephotoresist pattern that are caused by phase conflicts overlie theshallow trench isolations. The polysilicon layer is etched to form thepolysilicon gates. The photoresist layer is stripped away. Ions areimplanted to form lightly doped drains. Silicon nitride sidewall spacersare formed adjacent to the polysilicon gates and the shallow trenchisolations. Ions are implanted to form source and drain regions. Atitanium silicide layer is formed in the semiconductor substrate sourceand drain regions and in the polysilicon gates. An interlevel oxidelayer is deposited overlying the polysilicon gates, the source and drainregions, and the shallow trench isolations. The interlevel oxide layeris polished down to the top surface of the polysilicon gates. Aconductive layer is deposited overlying the polysilicon gates. Theconductive layer may comprise tungsten silicide or a combination oftitanium and titanium nitride. The conductive layer is patterned to forminterconnect lines from the polysilicon gates to other circuit featuresand to complete the fabrication of the polysilicon gate transistor inthe manufacture of the integrated circuit device.

Also in accordance with the objects of this invention, a method tofabricate an alternating aperture phase shift mask (AAPSM) for use informing polysilicon gate transistors in the manufacture of an integratedcircuit device is achieved. An opaque layer is provided overlying atransparent substrate. The opaque layer is patterned to form opaquefeatures that will overlie a portion of active areas of plannedtransistors. The opaque layer may be patterned to form either a clearfield or dark field mask. The opaque features will overlap theboundaries of the active areas onto the shallow trench isolations. Aphase shifting layer is formed in a portion of the transparent substrateadjacent to the opaque features on one side of each planned transistor.The phase shifting layer will overlie a portion of active areas forplanned transistors. The phase shifting layer will overlap theboundaries of the active areas onto the shallow trench isolations sothat unwanted features due to phase conflicts will overlie the shallowtrench isolations.

Also in accordance with the objects of this invention, an alternatingaperture phase shift mask (AAPSM) for use in forming polysilicon gatetransistors in the manufacture of an integrated circuit device isachieved. A patterned opaque layer overlies a transparent substrate. Thepatterned opaque layer features overlie a part of planned active areasof planned transistors. The patterned opaque layer features overlap theboundaries of the planned active areas onto the planned shallow trenchisolations. The patterned opaque layer is absent from all other areas. Apatterned phase shifting layer overlies the transparent substrateadjacent to the opaque features on one side of each planned transistor.The phase shifting layer overlies a part of planned active areas andoverlaps the boundaries of the planned active areas onto the plannedshallow trench isolations so that unwanted features due to phaseconflicts overlie the planned shallow trench isolations.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings forming a material part of thisdescription, there is shown:

FIGS. 1 through 7 schematically illustrate a partially completed priorart integrated circuit device using an AAPSM method.

FIGS. 8 through 23 schematically illustrate in cross-sectionalrepresentation a preferred embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The embodiment discloses the application of the present invention to theformation of polysilicon gate transistors in the manufacture of anintegrated circuit device. A method to form the alternating aperturephase shift mask is also disclosed. It should be clear to thoseexperienced in the art that the present invention can be applied andextended without deviating from the scope of the present invention.

Referring now particularly to FIG. 8, there is shown a cross section ofa partially completed integrated circuit device of the preferredembodiment. A semiconductor substrate 70, typically consisting ofmonocrystalline silicon, is provided. A gate dielectric layer 86 isformed overlying the semiconductor substrate 70. The gate dielectriclayer 86 serves as the gate oxide for the MOS transistors. The gatedielectric layer 86 is formed using either a thermal oxidation or alow-pressure, chemical vapor deposition (LPCVD) process. The gatedielectric layer 86 is formed to a thickness of between about 15Angstroms and 70 Angstroms. A polysilicon layer 90 is depositedoverlying the gate dielectric layer 86. The polysilicon layer 90 ispreferably deposited by a LPCVD process to a thickness of between about1,500 Angstroms and 2,500 Angstroms. The combined thickness of thepolysilicon layer 90 and the gate dielectric layer 86 will determine howfar the subsequently formed shallow trench isolations will extend abovethe surface of the semiconductor substrate 70.

Referring now to FIG. 9, the polysilicon layer 90, gate dielectric layer86, and semiconductor substrate 70 are patterned to form trenches forplanned shallow trench isolations. Following the trench etch, a trenchoxide layer 82 is deposited overlying the polysilicon layer 90 andfilling the trenches as shown in FIG. 10. The trench oxide layer 82 istypically comprised of silicon dioxide deposited using a high densityplasma (HDP) or LPCVD process.

Referring now to FIG. 11, an important part of the invention isillustrated. The trench oxide layer 82 is polished down to the topsurface of the polysilicon layer 90 to complete the shallow trenchisolations (STI) 82. Note that the amount by which the STI regions 82extend above the semiconductor substrate 70 is approximately thethickness of the polysilicon layer 90 deposited earlier. The extensionof the STI regions 82 above the semiconductor substrate 70 will allowthe AAPSM method of the present invention to pattern polysilicontransistor gates with a single mask exposure. The shallow trenchisolations 82 extend above the top surface of the semiconductorsubstrate 70 to a height of between about 1,000 Angstroms and 3,500Angstroms.

Referring now to FIG. 12, an important feature of the present inventionis shown. An alternating aperture phase shift mask (AAPSM) 100 isdepicted. The AAPSM 100 will be used in the formation of a memory cellcomprising six transistors. This mask is a clear field version of thelayout data. The AAPSM 100 is shown overlying active areas of p-welltype 104 and n-well type 106.

The mask 100 is comprised of a transparent substrate 102, opaquefeatures 112, and phase shift features 110. The transparent substrate102 is conventionally comprised of quartz. The opaque features 112conventionally comprise chrome that is patterned to correspond to theplacement of the planned polysilicon gates. Note that the opaquefeatures 112 define the polysilicon gates by crossing the active areas104, 106. The opaque features 112 overlap onto the non-active areaswhere shallow trench isolations are defined.

The phase shift features 110 are be formed by targeting a phase shift of180 degrees for the incident light compared to the transparent substrate102 is targeted. The phase shift features 110 may be formed by asubtractive process where a thickness of the transparent substrate 102is etched away. A thickness of between about 2,000 Angstroms and 3,000Angstroms of the transparent substrate 102 are removed in the phaseshift areas.

The phase shift features 110 are formed overlying the active areas ofthe planned transistors. The phase shift features 110 are formedadjacent to the opaque features 112 and cover one side of the plannedtransistors. This approach takes advantage of the principle of phaseshifting. When a photoresist layer is exposed through the AAPSM 100 thephase difference between the transparent substrate 102 and the phaseshift features 110 will cause the very narrow opaque features totransfer to the photoresist layer without overexposure. Using thistechnique, pattern widths as small as between about 0.07 microns and0.15 microns can be created in the photoresist layer and then etchedinto the polysilicon layer to form the transistor gates. In addition,because the phase shift features extend over the boundaries of theactive areas and onto the STI regions, any unwanted patterns that may beformed by unexposed photoresist due to conflicts between lighttraversing the transparent substrate 102 and light traversing the phaseshift features 110 will occur over the shallow trench isolation area,not over the polysilicon layer.

Referring now to FIG. 13, a dark field version of the same AAPSM mask101 is shown. In the dark field version, the polysilicon gate data iscombined with the non-active areas of the layout to form opaque features116 covering most of the circuit. Openings are formed in the opaquematerial for transparent substrate, or 0 degree phase shift, areas 114and for 180 degrees phase shift areas 110. The mask 101 is fabricatedwith the same materials and steps as the clear field version of FIG. 12.As in the clear field version, the dark field AAPSM mask 101 uses thephase shift to facilitate a sharp transfer of the opaque features 116onto the photoresist. In this case, the potential phase conflict iseliminated by the presence of the opaque features everywhere outside theactive areas.

Of course, the patterns outside of the active areas are still unwantedbut will have no adverse effect due to the presence of the extended STIregions.

Referring now to FIG. 14, the next cross sectional view of the preferredembodiment is shown. The photoresist layer is deposited and patternedusing the AAPSM mask. The photoresist is exposed using a lithographictool that utilizes a spectral wavelength of either about 193 nanometersor about 248 nanometers. The remaining photoresist layer afterdeveloping is shown with gate areas 120 defined. Note that someundesired photoresist 124 is left as a pattern due to the phaseconflict. The unwanted photoresist pattern 124 overlies the STI regions82.

Referring now to FIG. 15, a top view of the transistor is shown. Notehow the entire unwanted photoresist pattern 124 overlies the STI regions82.

Referring now to FIG. 16, the polysilicon layer 90 is etched to form thepolysilicon gates 90. The photoresist layer is stripped away. FIG. 17depicts a top view of the same transistor. Note that the polysilicongates 90 are confined to the active areas between the STI regions. Thepolysilicon transistor gates are etched to a width of between about 0.07microns and 0.15 microns.

Referring now to FIG. 18, ions are implanted to form lightly dopeddrains 128. Silicon nitride sidewall spacers 130 are formed, by aconventional process, adjacent to the polysilicon gates 90 and theshallow trench isolations 82. Ions are then implanted to form source anddrain regions 132. A metal silicide layer 134 comprising, for example,TiSi₂, CoSi₂, or NiSi, is formed in the semiconductor substrate sourceand drain regions 132 and on the polysilicon gates 90. The metalsilicide layer 134 reduces the contact resistance and the parasiticsource/drain resistance.

Referring now to FIG. 19, an interlevel oxide layer 138 is depositedoverlying the polysilicon gates 90, the source and drain regions, andthe shallow trench isolations 82. The interlevel oxide layer 138preferably comprises a boron and phosphorous doped silicon dioxide(BPSG) deposited by a CVD process to a thickness of between about 5,000Angstroms and 10,000 Angstroms. The interlevel oxide layer 138 ispolished down to the top surface of the polysilicon gates 90. A briefover polish is performed to create a slight recess of the interleveloxide layer 138.

Referring now to FIG. 20, a conductive layer 142 is deposited overlyingthe polysilicon gates 90. This conductive layer 142 may comprisetungsten silicide or a combination of titanium and titanium nitride. Iftungsten silicide is used, this layer may be formed by physical vapordeposition (PVD). If a combination titanium and titanium nitride layeris used, the titanium layer is deposited to a thickness of between about100 Angstroms and 200 Angstroms while the titanium nitride layer isdeposited to a thickness of between about 200 Angstroms and 1,000Angstroms.

Referring now to FIG. 21, the conductive layer 142 is patterned to forminterconnect lines from the polysilicon gates 90 to other circuitfeatures. This completes the fabrication of the transistor in themanufacture of the integrated circuit device. The conductive layer 142overlaps the polysilicon gate by between about 0.05 microns and 0.15microns to insure connectivity when a misalignment occurs. Theconductive layer 142 is only patterned in this way in areas having arelatively flat topology where depth of focus (DOF) issues areminimized. Since layer 142 has a thickness of less than about 1000Angstroms while the polysilicon gate has a thickness of about 2000Angstroms, there are less DOF issues for the contact and first metalpatterning because of relatively flat topology.

Referring now to FIG. 22, a top view of a representative transistorformed by the process of the present invention is shown. The conductivelayer 142 is shown overlying the polysilicon gate 90. The polysilicongate 90 crosses and overlies the active area 70 of the transistor. Theillustration depicts a large misalignment between the conductive layer142 and the polysilicon gate 90. However, there is no adverse effect dueto the misalignment. Compare this to the prior art example of FIG. 7,where a misalignment of this magnitude causes a highly resistive or openconnection to the gate. A significant advantage of the present inventionis demonstrated.

Referring now to FIG. 23, a final top view shows a memory cell 160created in the process of the present invention. Polysilicon transistorgates 172 are formed over the p-well active areas 164 and the n-wellactive areas 168. The conductive layer 176 connects the polysilicongates 172 to each other and to other possible components. Note how thememory cell 160 size is smaller than to that of the prior art memorycell illustrated in FIG. 5. The cell can be made smaller because theconductive layer 176 can be routed over the active areas 164, 168. Inthe prior art, the polysilicon layer used for-connectivity must berouted over STI regions and spaced from active areas.

As shown in the preferred embodiments, the present invention provides avery manufacturable process for fabricating a polysilicon gatetransistor using an alternating aperture phase shift mask (AAPSM) in anintegrated circuit device. In addition, a method to form the AAPSM maskis provided. The novel approach facilitates a process that providesseveral benefits. First, the method allows a 248 nanometer lithographictool to print fine lines down to 0.07 microns. Second, machine time isreduced by eliminating a double reticle change and exposure. Only asingle mask step is required to define the polysilicon gates. Third,since a separate layer is used for the interconnect, the layout can beshrunk. Fourth, device performance can be improved because smaller gatelengths are created. Fifth, the method is less sensitive to misalignmentbetween the polysilicon gate mask and the interconnect layer.

While the invention has been particularly shown and described withreference to the preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade without departing from the spirit and scope of the invention.

1. A method of fabricating a clear field alternating aperture phaseshifting mask (AAPSM) for use in forming polysilicon gate transistors inthe manufacture of an integrated circuit device comprising: providing anopaque layer overlying a transparent substrate; patterning said opaquelayer to form opaque features corresponding to planned transistor gateswherein said opaque features overlap boundaries of planned active areasonto planned shallow trench isolation areas and wherein said opaquelayer is removed from all other areas; and forming a phase shiftinglayer in a part of said transparent substrate adjacent to said opaquefeatures wherein said phase shifting layer overlies a part of saidplanned active areas on one side of each said planned transistors gate,wherein said phase shifting layer overlaps said boundaries of saidplanned active areas onto said planned shallow trench isolation areas,and wherein unwanted features caused by phase conflicts overlie saidplanned shallow trench isolations.
 2. The method according to claim 1wherein said phase shifting layer produces a 180 degree phase shift withrespect to said transparent substrate.
 3. The method according to claim1 wherein said step of forming said phase shifting layer is by asubtractive process wherein said transparent substrate is etched.
 4. Amethod of fabricating a dark field alternating aperture phase shiftingmask (AAPSM) for use in forming polysilicon gate transistors in themanufacture of an integrated circuit device comprising: providing anopaque layer overlying a transparent substrate; patterning said opaquelayer to remove said opaque layer from planned active areas such thatexposed said transparent substrate overlaps boundaries of said plannedactive areas onto planned shallow trench isolation areas and whereinsaid opaque layer remains in all other non-active areas to form opaquefeatures corresponding to planned transistor gates; and forming a phaseshifting layer overlying a part of said transparent substrate adjacentto said opaque features wherein said phase shifting layer will overlie apart of said planned active areas on one side of each said plannedtransistors gate, wherein said phase shifting layer will overlap saidboundaries of said planned active areas onto said planned shallow trenchisolation areas, and wherein unwanted features caused by phase conflictsoverlie said planned shallow trench isolations.
 5. The method accordingto claim 4 wherein said phase shifting layer produces a 180 degree phaseshift with respect to said transparent substrate.
 6. The methodaccording to claim 4 wherein said step of forming said phase shiftinglayer is by a subtractive process wherein said transparent substrate isetched.
 7. An alternating aperture phase shift mask for use infabricating polysilicon gate transistors in the manufacture of anintegrated circuit device comprising: a transparent substrate; apatterned opaque layer overlying said transparent substrate wherein saidpatterned opaque layer features correspond to planned transistor gatesand overlap boundaries of planned active areas onto planned shallowtrench isolation areas and wherein said patterned opaque layer is absentfrom all other areas; and a patterned phase shifting layer overlying apart of said transparent substrate adjacent to said patterned opaquelayer features wherein said phase shifting layer overlies a part of saidplanned active areas on one side of each said planned transistor gate,wherein said phase shifting layer overlaps said boundaries of saidplanned active areas onto said planned shallow trench isolation areas,and wherein unwanted features caused by phase conflicts overlie saidplanned shallow trench isolations.
 8. The method according to claim 7wherein said patterned phase shifting layer produces a 180 degree phaseshift with respect to said transparent substrate.